Processing techniques such as ion implantation provide the ability to process substrates using a variety of species. In ion implantation systems, a variety of ion sources are available including solid, liquid, and gas-based ion sources. For many ion species, a variety of gaseous precursors are available to serve as a source of the material to be used to process the substrate. However, for some ionic species gaseous precursors do not exist, or those that do exist yield low beam current. For those ionic species lacking an effective gaseous precursor, liquid and/or powder precursors may be available, but such precursors entail the use of a vaporizer in order to produce gas phase species for the ion source from which ions are to be extracted.
An alternative approach to the above precursors is the use of a solid target that can be inserted into an ion source chamber. In such configuration, a plasma may be generated using chosen ion species in the ion source chamber, such as inert gas ions. The solid target may act as a sputtering target for the plasma ions, wherein the plasma ions are accelerated toward the target and sputter atoms or groups of atoms from the target. However, if the target is a dielectric material, a dc bias applied to the target will be ineffective in biasing the target such that the effective potential drop between plasma and target is equivalent to the difference in plasma potential and floating self bias potential that develops on the surface of the dielectric target. This potential, equivalent in volts to the ion energy in eV for singly charged ions, is typically in the range of several volts to several tens of volts, resulting in ions striking the target with energies that may be less than 100 eV. Because the sputter yield for most materials is low in this energy range, it may be desirable to employ an etchant gas in conjunction with the plasma to increase the yield rate of target atoms into the gas phase.
Gallium ions have long been used in focused ion beam processing in which the gallium ions are derived from a liquid source. However, this may not be suitable for systems requiring a high current density over a relatively larger area. In the case of solid targets, a solid Ga target cannot be readily used for an ion source due to the low melting temperature of the elemental metal. In view of the above, it will be apparent that improvements in gallium ion sources are needed.